1. Field of the Invention
The present invention relates to an image reading apparatus for scanning and exposing an original image, and photoelectrically converting the image using an image sensing element and, more particularly, to an apparatus for reading a tone image such as a photograph.
2. Related Background Art
Conventional image reading apparatuses using image sensing elements such as CCDs have been available.
FIG. 1 is a block diagram of an image reading apparatus of this type. In FIG. 1, light reflected by, e.g., an original placed on an original table, or projection light through a microfilm is scanned and photoelectrically converted by an image sensing unit 61 using an image sensing element such as a CCD, and an electrical signal is quantized into multi-value data by an A/D converter unit 62.
The multi-value quantized data is supplied to the next tone processing unit 63, and is converted by, e.g., a dither method, into binary data (e.g., 1/0) which allows reproduction of a tone image. The binary data is output from the reading apparatus.
The arrangement shown in FIG. 1 faithfully reads an original image having a low contrast. Therefore, output data is reproduced to have a low contrast as in the original image, thus posing a problem.
As shown in FIG. 2, another arrangement has been proposed wherein a .gamma. correction unit 64 is arranged between the A/D converter unit 62 and the tone processing unit 63.
In FIG. 2, multi-value quantized data output from the A/D converter unit 62 is input to the next .gamma. correction unit 9 to correct a difference between photoelectric conversion characteristics of the image sensing element such as a CCD and human tone visual recognition characteristics. FIGS. 3, 4, and 5 show this state.
FIG. 3 represents photoelectric conversion characteristics of the image sensing element. In FIG. 3, the abscissa corresponds to a scale of density which linearly changes with respect to human tone visual recognition characteristics. In this manner, since the photoelectric conversion characteristics of the image sensing element are normally different from the human tone visual recognition characteristics, a .gamma. correction table 65 shown in FIG. 4 is used to perform correction (.gamma. correction), so that an output D' linearly changes with respect to the human tone visual recognition characteristics, as shown in FIG. 5.
The .gamma.-corrected signal is supplied to the next tone processing unit 63, and is converted by, e.g., a dither method, into binary data (e.g., 1/0) which allows reproduction of a tone image, as shown in FIG. 2. The binary data is output from the reading apparatus.
The arrangement shown in FIG. 2 faithfully outputs an original image having a low contrast. Therefore, output data is reproduced to have a low contrast as in the original image, thus posing a problem.
As shown in FIG. 6, still another arrangement which has a tone processing unit 67 using a dither method to reproduce a pseudo halftone image on the basis of multi-value quantized data from the A/D converter unit 62 has been proposed.
Multi-value quantized data obtained by multi-value quantization by the A/D converter unit 62 is supplied to the tone processing unit 67, and is converted by, e.g., a dither method, into binary data (e.g., 1/0) which allows reproduction of a tone image. More specifically, dot patterns having dot densities proportional to density data of an image are sequentially read out from a dither pattern memory 66 storing dot patterns shown in FIG. 7, and the readout patterns are output.
However, a tone of an original whose image entirely has a low density and a low contrast, as shown in FIG. 39, or of an original whose image entirely has a high density and a low contrast, as shown in FIG. 40 is faithfully reproduced. Therefore, a reproduced image has a low contrast as in the original image.
The above-mentioned problems also occur when an image of a normal document, book, or the like is read, and are conspicuous when an image recorded on a film is to be read.
A density and a contrast of an image recorded on a film largely depend on various conditions such as photographing conditions, development conditions, and the like as well as a type of film. Therefore, this implies that images have quite different densities and contrasts in units of films to be read.
Therefore, when an image recorded on a film is to be read, reading cannot be satisfactorily attained unless the above-mentioned problems are solved.